A high-intensity discharge lamp (HID lamp)—such as an automotive Xenon lamp for a front beam—may be realised together with an ignitor, i.e. the lamp is mounted to the ignitor housing and is electrically connected to components inside the ignitor housing. Lamp and ignitor may be realised as a single product, with the lamp mounted directly to a housing containing the ignitor components. The ignitor housing generally has a connection interface for connecting a number of electrical leads to an electronic lamp driver. The ignitor generally comprises a transformer, an ignition capacitor, a voltage limiter such as a clipping diode, a discharge resistor and a spark gap. The purpose of the ignitor is to establish a discharge arc across the tips of the electrodes inside the discharge chamber of the lamp (also referred to as a “burner”) so that normal or steady-state operation can begin. A known type of ignitor such as an asymmetrical ignitor comprises a pair of input terminals for applying the ignition voltage, and a pair of terminals for applying a drive voltage to the lamp during steady-state operation.
The lamp driver is used to regulate the lamp power. In some situations, for example night-time driving outside of a built-up area, more light on the road is desirable for safety reasons. To this end, some electronic lamp drivers for automotive HID lamps are designed and manufactured to be able to increase the lamp power in such a situation. Furthermore, lamps may be designed to consume more power, for example in an upper power limit allowed by the appropriate regulation. Increasing the lamp power for whatever reason leads to an increase in the temperature inside the ignitor housing, and the components of the ignitor may become damaged as a result. For example, if the lamp is driven at a boosted or higher power that is greater than the lamp's nominal power, the ignitor components are subjected to increased thermal stress. The heat originates from the burner, which is physically very close to the ignitor housing. It is not uncommon for the ignitor components to reach temperatures close to 150° C., which is the usual specification limit for such components. All of the components mentioned above can fail due to overheating. Clearly, the lifetime of the components and therefore of the ignitor are reduced significantly if the ignitor is allowed to become too hot. Other problems associated with a too-hot ignitor are that re-ignition of an overheated ignitor (‘hot re-strike’) can fail, or high-voltage isolation of the ignitor can be reduced and can cause re-ignition failures. Another factor that encourages the development of high temperatures in the ignitor is the trend towards more compact headlamp assemblies.
In one approach to dealing with this problem, the power applied to the lamp is reduced in order to lower the temperature in the ignitor. However, in order to regulate the power, some information about the ignitor temperature is required. Therefore, in one known approach, cf. US20050067979A1, EP2244537A2, US6072283A, and WO2010136918A1, a temperature sensor can be arranged inside the ignitor housing and connected by electrical leads to an external module that monitors the temperature development and regulates the lamp power accordingly. However, this solution requires an alteration to an existing interface between the ignitor and the lamp driver in order to accommodate the additional electrical leads, and is therefore unattractive from a commercial point of view. In an alternative approach, heat-dissipating elements can be mounted to the outside of the ignitor housing in order to draw some heat away from the ignitor and its components, and/or a ventilator may be used to blow cool air over the ignitor. However, such elements are bulky and therefore unattractive and also add to the overall cost of a lighting arrangement with such an ignitor.
Therefore, it is an object of the invention to provide an improved way of operating a high-intensity discharge lamp that avoids the problems outlined above.